Active suspension system for an automobile

ABSTRACT

An automotive active suspension system according to this invention uses valves of a simplified structure so that the availability of installing space is improved and energy consumption is reduced by decreasing the quantity of the returned fluid. 
     This invention comprises a hydraulic pressure creating/supply apparatus; hydraulic actuators arranged between each wheel and body; wheel pressure control valves for proportionally controlling the pressure applied to each hydraulic actuator; main line pressure control valves arranged between said hydraulic pressure creating/supply apparatus and said wheel pressure control valves to control the main line pressure supplied through main lines; a predetermined pressure-maintaining valve which is located in a return line to said hydraulic pressure creating/supply apparatus from said wheel pressure control valves to receive a pilot pressure passing through said main line pressure control valves; and a relief valve which is located in a line connecting the upstream line of said main line pressure control valves and the downstream line of said predetermined pressure-maintaining valve in order that a bypass line of said main line pressure control valve is connected to the downstream line of said relief valve.

BACKGROUND OF THE INVENTION

This invention relates to an active suspension system for an automobile,and more particularly, to an active suspension system for an automobilewhich can maintain the vehicle-positioning control andcontrollability/stability at both the normal driving mode and abnormaltrouble mode by continuously controlling fluid pressure of an hydraulicactuator utilizing a wheel pressure control valve and a predeterminedpressure-maintaining valve.

An automotive active suspension system is a system which can activelycontrol suspension itself, by leveling the vehicle or tuning thesuspension according to the conditions of road surfaces or drivingcondition of an automobile utilizing a pressure source which providesforce for eliminating the force of inertia applied to the vehicle bodyor external force being transmitted from the road surface. A typicalexample of such an automotive active suspension system is disclosed inU.S. Pat. No. 5,085,460, and its schematic hydraulic circuit is shown inFIG. 8.

The system has a hydraulic pump 1 as a pressure source, actuators 15arranged at each wheel, control valve units 25 as means for controllingpressure of the actuators 15, and a pressure-maintaining unit 24 forcontrolling pressure.

Between a pressure accumulator 3 for absorbing pressure pulsationscreated by the hydraulic pump 1 which is a pressure source, and a reliefvalve 6 for setting the system pressure a filter 5 and a check valve 4for preventing the filter 5 from being damaged are provided, and in thereturn line a hydraulic oil cooler 21 and a hydraulic oil tank 23 areprovided.

The actuators 15, which are arranged corresponding with each wheel,include a sub-accumulator 13 and a damping orifice 14, and the controlvalve units 25 are divided into two for the front wheels and the rearwheels. There are two control valves 20 in the control valve unit 25 forthe rear wheels, which control each actuator 15. There is a mainpressure accumulator 11 in the supply line of the control valves 20, andthere are a return pressure accumulator 16 and damping orifices 17,19 inthe return line.

The pressure-maintaining unit 24 as a pressure control part includes anorifice 7 and integral flow control valve 8 for preventing the vehicularheight level from suddenly being changed right after the enginestarting, a safety valve 10 for preventing any sudden change of thevehicular height level and for maintaining a predetermined vehicularheight level if trouble occurs in the electric circuit, and apressure-maintaining check valve 18 and a relief valve 6. The pressurecontrol valve 20, a proportional electronic type, which is arranged on afluid line reaching the actuators 15, controls the posture of a vehicleby supplying or exhausting the hydraulic fluid to/from the actuators 15,for example, control outputs IF, IF from a control unit based ondetecting output X of a positional acceleration sensor are transmittedto each proportional solenoid of the control valves 20, whereby thepressure control valve 20 controls the pressure of each actuator 15.

Such a hydraulic circuit as above operates as follows;

The relief valve 6,maintains the system pressure constant, and the flowcontrol valve 8, including an ON/OFF solenoid 8S which is operated bythe orifice 7 and control signal IFV, has the duty of preventing anysudden change of a vehicular height level right after the enginestarting.

The pressure-maintaining check valve 18 is opened/closed to maintain apredetermined vehicular height level, by controlling pilot pressure ofthe fail safe valve 10 which is operated by the ON/OFF solenoid 10Swhich is operated by control signal IFS of the control unit 27 based ondetecting signal Y of a fail detector 29.

The prevention of a sudden change of a vehicular height level, which iscaused by variation of the return pressure according to the change ofopening degree of the pressure-maintaining check valve 18 after theengine stops, is carried out by the damping orifice 17, the returnpressure accumulator 16, and the main pressure accumulator 11. The mainpressure accumulator 11 fills up the quantity of shortage of supplyfluid from the hydraulic pump 1 in case that the actuators 15 urgentlyrequire much fluid, absorbs the variation of the supply pressure, andprevents the sudden change of a vehicular height level by smoothlyreducing the return pressure according to leakage during constantlymaintaining the return pressure by the operation of the check valve 18after the engine stops. The return pressure accumulator 16 absorbs thepressure pulsations in the return line.

The sub-accumulator 15 and damping orifice 14 of each actuator 15,arranged to corresponding with each wheel, absorb the frequencyvibrations.

However, in such a conventional hydraulic circuit, the flow controlvalve 8 having the ON/OFF solenoid 8S and the fail safe valve 10 havingthe ON/OFF solenoid 10S are very complex in their structure, and theyconsist of numerous parts, so that their requiring manufacturing is verydifficult.

Further, because the above valves 8,10 are of a directly moving spoolvalve type, a big solenoid is used to directly control the spool.Accordingly, their dimensions become large. As a result, they require alarge installation space.

On the other hand, the conventional system causes much energy lossbecause the return pressure is increased by the damping orifice 19arranged on the return line 26R in the normal control mode.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an automotive activesuspension system which improves the availability of the installationspace in a vehicle by using one control valve compared with aconventional controlling system which uses two valves.

It is another object of this invention to provide an automotive activesuspension system which reduces energy consumption and noise byreleasing the relief pressure at a low pressure instead of a highpressure.

In order to achieve the above purposes, an automotive active suspensionsystem according to this invention includes:

a hydraulic pressure creating/supply apparatus;

hydraulic actuators arranged between each wheel and body;

wheel pressure control valves for proportionally controlling thepressure applied to each hydraulic actuator;

main line pressure control valves arranged between the hydraulicpressure creating/supply apparatus and the wheel pressure control valvesto control the main line pressure supplied through the main lines;

a predetermined pressure-maintaining valve which is located in a returnline to the hydraulic pressure creating/supply apparatus from the wheelpressure control valves to receive a pilot pressure passing through themain line pressure control valves; and

a relief valve which is located in a line connecting the upstream lineof the main line pressure control valves and the downstream line of thepredetermined pressure-maintaining valve in order that a bypass line ofthe main line pressure control valve is connected to the downstream lineof the relief valve.

In such an automotive active suspension system, the main line pressurecontrol valves makes the system line, connected to the hydraulicpressure creating apparatus, communicate to the main line in the normaldriving mode, and make it communicate to the bypass line in the troublemode and the engine stop mode.

Further, in this system, the predetermined pressure-maintaining valve isopened in the normal driving mode, and it is closed in the trouble modeand the engine off mode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram of an automotive active suspensionsystem according to this invention;

FIG. 2 is a cross-sectional view of a main line pressure control valveused in this invention;

FIG. 3 is a cross-sectional view of a predetermined pressure-maintainingvalve used in this invention;

FIG. 4 is a cross-sectional view of a relief valve used in thisinvention;

FIG. 5 is a cross-sectional view of a wheel proportional pressurecontrol valve used in this invention;

FIG. 6 is a graph showing pressure variation in the hydraulic circuit inthe normal driving mode;

FIG. 7 is a graph showing pressure variation in the hydraulic circuit inthe trouble mode; and

FIG. 8 is a hydraulic circuit diagram of an conventional automotiveactive suspension system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a hydraulic circuit diagram according to an embodiment of thisinvention.

This active suspension system comprises a hydraulic pressurecreating/supply apparatus 380, hydraulic actuators 30FR, 30RR, 30RL 30RRand 30FL (hereinafter designated Numeral. 30) arranged between thevehicle body(not shown) and the respective wheels, a front wheel valveunit 39F and a rear wheel valve unit 39R which are arranged between thehydraulic pressure creating/supply apparatus 380 and each hydraulicactuator 30 to connect or disconnect the pressurized fluid from thehydraulic pressure creating/supply apparatus 380 to each hydraulicactuator 30, or from each hydraulic actuator 30 to the hydraulicpressure creating/supply apparatus 380.

The hydraulic pressure creating/supply apparatus 380 includes variablevolume type hydraulic pump 38, a pressure accumulator 3 located at theoutput of the pump 38 to absorb the pumping pulsations, a hydraulic oiltank 23 having a filter and a check valve for preventing damage of thefilter which is located at the inlet of the pump 38.

Hydraulic actuators 30 include a piston damping valve 30P, a bodydamping valve 34D 30D, a main pressure accumulator 30M having a cylinderpressure chamber 30C, and a sub-accumulator 30A communicating with themain pressure accumulator 30M.

The front wheel valve unit 39F includes a filter 5, a pressure controlvalve 35 for controlling the pressure in a supply line Li, and pressurecontrol valves 20FR, 20FL of the front wheel side for controlling thefluid pressure being supplied to each hydraulic actuator 30, which areall located in the supply line Li.

A check valve 4 for preventing damage of the filter 5 is arranged inparallel, and a main check valve 9 is arranged downstream of thepressure control 35, and a filter 33F is arranged upstream of thepressure control valves 20FR,20FL, and a pressure accumulator 33F, whichis accumulating the pressure of the supply line Li, is arranged upstreamof the filter 33F.

On the other hand, in the front wheel valve unit 39F, a valve 37 formaintaining a predetermined pressure for setting a vehicular heightlevel is arranged on a return line Lo extending to the hydraulic oiltank 23 from each hydraulic control valve 20FR,20FL, and a hydraulic oilcooler 21 is arranged near the hydraulic oil tank 23. A return pressureaccumulator 16F of the front wheel side is arranged between eachhydraulic pressure control valve 20FR,20FL and the predeterminedpressure-maintaining valve 37.

Further, a relief valve 6 for setting a pressure of a system line Lsbetween the pump 38 and the pressure control valve 35 in the supply lineLi is arranged at the line connecting the upstream of the predeterminedpressure-maintaining valve 37, and a check valve 34 for preventing thereturn pressure from being abnormal and an air vent 34B arranged inparallel with the valve 34 are provided at a line between the supplyline Li and the return line Lo.

Hereinafter, for a clear explanation of the supply line Li, the linebetween the hydraulic pressure pump 38 and the pressure control valve 35is named as a system line Ls, the line between the pressure controlvalve 35 and the pressure control valves 20FR,20FL of the front wheelside is named as a front wheel main line Ls1, the line between thepressure control valve 35 and the pressure control valves 20RR,20RL ofthe rear wheel side is named as a rear wheel main line Ls2, and the linebetween each wheel pressure control valve 20FR,20FL,20RR,20RL and eachhydraulic actuator 30 is named as a control line Lc.

Also, on the return line Lo, the line between the pressure controlvalves 20FR,20FL of the front wheel side and the predeterminedpressure-maintaining valve 37 is named as a front wheel return line Lr1,the line between the pressure control valves 20RR,20RL of the rear wheelside and the predetermined pressure-maintaining valve 37 is named as arear wheel return line Lr2, the line between the predeterminedpressure-maintaining valve 37 and the hydraulic oil tank 23 is named asa tank line Lt, the line which is directly connected to the tank line Ltfrom the pressure control valve 35 is named as a bypass line Lb, and theline which is connected to the predetermined pressure-maintaining valve37 after being branched from the downstream line of the pressure controlvalve 35 is named as a pilot line 26P.

The rear wheel valve unit 39K consists of the rear wheel side pressurecontrol valves 20RR,20RL which control the fluid pressure transmitted toeach actuator 30, and a filter 33R is provided at the upstream side ofthe pressure control valves 20RR,20RL, and at the upstream side of thefilter 33R a pressure accumulator 33R is installed which accumulates thefluid pressure of the supply line Li.

Further, a check valve 34 for preventing the return pressure from beingabnormal and an air vent 34B arranged in parallel to the valve 34 areprovided at the line connecting the rear wheel main line Ls2 to the rearwheel return line Lr2.

A rear wheel side return pressure accumulator 16R is also installedbetween each, pressure control valve 20RR,20RL and the predeterminedpressure-maintaining valve 37.

In such a hydraulic circuit according to this invention, by means of acreating signal X from a positional acceleration sensor 28 and acreating signal Y from a fail detector 29 in order to control thepressure of the hydraulic pump 38 and the pressure of each hydraulicactuator 30, a control unit 32 sends a control signal IBY to theproportional solenoid 35S of the main line pressure control valve 35,and sends control signals IFL, IFR, IRR, IRL (hereinafter named as Ic)to the proportional solenoids 20S of each wheel pressure control valve20FR,20FL,20RR,20RL, thereby maintaining an optimum posture of avehicle.

As shown in FIG. 2, the main line pressure control valve 35 comprises abody 55, a spool 57 capable of sliding in the body 55, and a spoolsleeve 56 arranged at the outside of the spool 57. 0n one end of thespool 57 a spring 59 is mounted which applies a constant elastic forceset by the regulation of a spring jig 58, to the spool 57, and a plunger51 in the proportional solenoid 35S which is operated by the controlsignal IBY extends towards the other end of the spool 57.

FIG. 2 shows the ON position of the proportional solenoid 35S(namely,the normal condition during the engine driving), in which the plunger 51pushes the spool 57 to the upside so that the system line L and thefront/rear main lines Lr1,Ls2 communicate with each other. On the otherhand, in the OFF position of the proportional solenoid 35S(namely, theengine stopped, or abnormal condition during the engine driving) theplunger 51 moves downward, and at the same time the spool 57 is moveddownward by the spring 59, so that the system line Ls communicates withthe bypass line Lb.

As shown in FIG. 3, the predetermined pressure-maintaining valve 37comprises a body 70, a sleeve 71 arranged in the interior of the body70, a pilot spool 73 so as to be capable of sliding in the sleeve 71,and a pilot poppet 72 which is arranged in contacting position with oneend of the pilot spool 73 so that the front/rear wheel return linesLr1,Lr2 can communicate with the tank line Lt.

The predetermined pressure-maintaining valve 37 further includes aspring 74 for elastically supporting the other end of the pilot poppet72 and a regulating jig 75 which can regulate the elastic force of thespring 74 in order to apply a predetermined cracking force to the pilotpoppet 72.

The predetermined pressure-maintaining valve forms such a structure thatthe pilot poppet 72 moves downward so that the front/rear wheel returnlines Lr1,Lr2 communicate with tank line Lt, in the case that resultantforces of the fluid pressure in the pilot line 26P plus the pressure inthe front/rear wheel return lines Lr1,Lr2 exceed the cracking force as apredetermined force of the spring 74.

As shown in FIG. 4 illustrating the relief valve 6, the relief valve 6comprises a body 60, upper/lower sleeves 61U,61L, and a poppet 62arranged so as to be capable of sliding in the lower sleeve 61L. Therelief valve 6 further includes a spring 63, which is arranged at thelower side of the poppet 62 to apply the elastic force to the poppet 62so as for the poppet 62 to contact the upper sleeve 61U, and aregulating jig 64 for regulating the elastic force of the spring 63 soas to apply a predetermined cracking force to the poppet 62.

In the lower sleeve 61L an orifice 65 is provided to send fluid into thechamber 66 to the tank line Lt. The relief valve 6 has such a structurethat the poppet 62 moves downward so as for the system line Ls tocommunicate with the tank line Lt in the case that the fluid pressurefrom the system line Ls exceeds a predetermined pressure of the spring63. If the poppet 62 moves downward, the pressurized fluid in thechamber 66 is discharged to the tank line Lt through the orifice 65 forinducing resistance of the fluid, whereby the chattering phenomena by asudden change of the pressure in the chamber 66 is prevented.

As shown in FIG. 5, the wheel pressure control valve 20 includes a pilotvalve 20P which is operated by control signal Ic sent from the controlunit 32 in order that body vibrations according to the condition of roadsurfaces reacts in the range of low frequency, and a main valve 20Mwhich mechanically operates in order that the above vibrations react inthe range of medium frequency.

The pilot valve 20P comprises a body 40, an outer sleeve 54 having areturn orifice 48 which communicates with the return lines Lr1,Lr2, andan inner sleeve 51 having both a supply orifice 52 communicating withthe main lines Ls,Ls2 and a pilot orifice 47 to send fluid to the mainvalve 20M. The pilot valve also includes a poppet 50 which is arrangedin the condition of being able to slide in the outer sleeve 54 and thehead portion of which can block one end of a poppet passage 53. Further,the pilot valve 20P includes a plunger 49 in the proportional solenoid20S which is operated by the control signal Ic, to push the rear portionof the poppet 50.

The main valve 20M comprises the common body 40 with the pilot valve20P, a spool sleeve 42 mounted in the body 40, and a spool 41 arrangedso as to be able to slide in the spool sleeve 42. The spool sleeve 42includes a port communicating with the main lines Lr1,Ls2, a portcommunicating with the control line Lc, and a port communicating withthe return lines Lr1,Lr2.

The spool 41 has a pilot chamber 55P in its lower portion, whichcommunicates with the pilot orifice 47, a return chamber 55F in itsupper portion, which contains a spring 45 compressed by a jig 43, and areturn orifice 46 which connects the return chamber 55F to the outsideof the spool 41.

In such a wheel pressure control valve 20, in case that the bodyvibrations are in the range of low frequency, the force of theproportional solenoid 20S increasing linearly, the plunger 49 graduallymoves left, and then the poppet 50 gradually closes the poppet passage53. With increasing the pressurized fluid in the passage 53, thepressure in the pilot orifice 47 increases gradually to raise the spool41 little by little. This causes that the opening degree of the portscommunicating the main lines Lr1,Ls2 with the control line Lc to becomegradually larger. Consequently, the quantity of the fluid flowing to thecontrol line Lc increases.

On the other hand, in the case that the wheels encounter a projectedpart on the road's surface, namely the wheels bound and the pressure ofthe compression chamber 30C in the hydraulic actuator 30 increases. Atthis time, with an increase of the fluid pressure in the control lineLc, the fluid flows to the return chamber 55F through the return orifice46, so that the spool 41 is moved downward by the increased pressure,whereby the control line Lc communicates with the return lines Lr1,Lr2.Consequently, the fluid in the control line Lc flows to the return linesLr1,Lr2, so that the pressure of the compression chamber 30C in thehydraulic actuator 30 decreases, thereby maintaining a predeterminedvehicular height level.

On the contrary, in the case that the wheels pass an indented part inthe road, namely the wheels rebound and the pressure of the compressionchamber 30C in the hydraulic actuator 30 decreases. At this time, with adecrease of the fluid pressure in the control line Lc, the fluid in thereturn chamber 55F flows to the control line Lc through the returnorifice 46, so that the spool 41 is moved upward, whereby the controlline Lc communicates with the main lines Lr1,Ls2. Consequently, thefluid in the main lines Lr1,Ls2 flows to the control line Lc, so thatthe pressure of the compression chamber 30C in the hydraulic actuator 30increases, thereby maintaining a predetermined vehicular height level.

The hydraulic circuit of this invention constructed as above operates asfollows:

FIG. 6 is a chart illustrating the variation of the fluid pressure ineach hydraulic line in the condition of the engine stopped, right afterthe engine starting, and in various modes during normal driving. Whilethe engine is stopped (0-TO), with the neutral position of the spool 41in the wheel pressure control valve 20, the main lines Lr1,Ls2 and thereturn lines Lr1,Lr2 communicate with the control line Lc connecting tothe hydraulic actuator 30. The front/rear wheel main lines Lr1,Ls2 areblocked from the system line Ls by the spool 57 in the main linepressure control valve 35. Also, the predetermined pressure-maintainingvalve 37 closes. Therefore, the fluid pressure in both the main linesLr1,Ls2 and the return lines Lr1,Lr2 becomes the same pressure as apressure P3 for maintaining a predetermined vehicular height level inthe control line Lc. In this condition, the fluid pressure in both thesystem line Ls and the tank line Lt is zero.

Right after the engine starting(TO), with gradual pumping of thehydraulic pump 38, the fluid pressure in the system line Ls becomes P1,and the fluid flows to the bypass line Lb through the main line pressurecontrol valve 35, whereby the pressure of the bypass line Lb alsobecomes P1.

The fluid pressure in the system line Ls keeps P1 for the time T1. Evenat this time, the fluid pressure in the main lines Lr1,Ls2 and thereturn lines Lr1,Lr2 continuously maintains the pressure P3 formaintaining the vehicular height level because not only the system lineLb in the main line pressure control valve 35 is blocked from thefront/rear wheel main lines Lr1,Ls2, but also the predeterminedpressure-maintaining valve 37 continues to remain closed.

For the time T2, the fluid pressure in the system line Ls graduallyrises to the pressure P3 for maintaining the vehicular height level.From the time T1, with the control signals IBY which are sent to theproportional solenoid 35S in the main line pressure control valve 35,increasing linearly, the system line Ls starts to communicate with thefront/rear wheel main lines Lr1,Ls2, so that the fluid pressure in thesystem line Ls increases with the fluid pressure in the front/rear wheelmain lines Lr1,Ls2 which was maintaining the pressure P3 for maintainingthe vehicular height level. At the same time, the predeterminedpressure-maintaining valve 37 starts to be opened gradually by thepressure of the fluid passing through the pilot line 26P. Consequently,the fluid pressure in the return lines Lr1,Lr2 decreases gradually.

For the time T3, the fluid pressure in the main lines Lr1,Ls2 increasesgradually to reach the pilot pressure Pp where the predeterminedpressure-maintaining valve 37 is opened completely. Accordingly, thefluid pressure in the return lines Lr1,Lr2 decreases to low pressure P1.

For the time T4, by the operation of the hydraulic pump 38, the fluidpressure in the system line Ls increases to the system pressure P2, andthe main lines Lr1,Ls2 in the main line pressure control valve 35communicate with the system line Ls completely, whereby the fluidpressure in the main lines Lr1,Ls2 also maintains at P2.

While the normal driving from T4 to T5, the fluid pressure in the systemline Ls and the main lines Lr1,Ls2 maintains P2, while the fluidpressure in the return lines Lr1,Lr2 and the tank line Lt maintains P1.In this condition, the fluid pressure in the control line Lc keeps thepressure P3 for maintaining the vehicular height level.

Under the condition that each line keeps the respective pressures asabove, by the control signal Ic created by the control unit 32 when thevehicular posture changes, the wheel pressure control valve 20 isoperated, whereby the vehicle can be driving in the condition ofmaintaining the vehicular height level.

The operation of the wheel pressure control valve 20 will not bedescribed because it has been described already.

At the time T5 when the hydraulic pump 38 stops, the fluid pressure inthe system line Ls and the tank line Lt become zero. At the same time,the fluid pressure in the main lines Lr1,Ls2 starts to decreasegradually.

At the time T6, the main lines Lr1,Ls2 which have been communicatingwith the system line Ls in the main line pressure control valve 35,start to block each other. At the same time, with the fluid pressure inthe main lines Lr1,Ls2 becoming the pilot pressure Pp, the predeterminedpressure-maintaining valve 37, which has been communicating with thepilot line 26P, starts to close gradually.

At the time T7, the main lines Lr1,Ls2, which have been communicatingwith the system line Ls in the main line pressure control valve 35, areblocked completely. The return lines Lr1,Lr2, and the tank line Lt,which has been communicating with each other by the predeterminedpressure-maintaining valve 37, are also blocked completely. As a result,both the pressure of the main lines Lr1,Ls2 and the pressure of thereturn lines Lr1,Lr2 keep the pressure of the control line Lc and thepressure P3 for maintaining the vehicular height level.

FIG. 7 is a chart illustrating variation of the fluid pressure in eachhydraulic line when any trouble happens during normal driving. If anysystem trouble occurs at the time T5, namely during normal driving, thecontrol signal IBY from the control unit 32, which is sent to theproportional solenoid 35S of the main line pressure control valve 35, isbroken. At this time, the system line Ls, which communicates with themain lines Lr1,Ls2 in the main line pressure control valve 35,communicate with the bypass line Lb. Accordingly, the fluid pressure inthe main lines Lr1,Ls2 decreases to keep the pressure P3 for maintainingthe vehicular height level. At the same time, the pressure of the fluid,flowing to the predetermined pressure-maintaining valve 37 through thepilot line 26P, decreases, and therefore the return lines Lr1,Lr2 andthe tank line Lt in the predetermined pressure-maintaining valve 37, aredisconnected completely from each other. At this time, the pressure ofthe return lines Lr1,Lr2, which has been maintained as P1, rises to P3by the pressure of the main lines Lr1,Ls2. Consequently, the pressure ofthe main lines Lr1,Ls2 and the return lines Lr1,Lr2 remains at P3 formaintaining the vehicular height level equal to the pressure of thecontrol line Lc.

Even in this condition, the hydraulic pump 38 continues to operate inorder to move the vehicle. But, the pressurized fluid is relieved to thelow pressure P1 and then returned to the hydraulic oil tank 23 throughthe tank line Lt because the system line Ls of the main line pressurecontrol valve 35 is connected to the bypass line Lb.

As explained above in detail, this invention has a duty of apredetermined vehicular height level in the system trouble mode andparking mode and for preventing any sudden change of the vehicularheight level right after the engine starting/stopping, by thecombination of both the predetermined pressure-maintaining valve and themain line pressure-maintaining valve which all have a simple structure.

Further, because the main line pressure control valve, the proportionalpressure control valve, the relief valve, and the predeterminedpressure-maintaining valve hold some parts in common and their structureis similar, worn parts in the working process decrease, their assemblyis easier, and the manufacturing cost is reduced.

Further, by adapting the method of completely shutting off the main linepressure control valve in the normal control mode, the returned fluid,that is, the consumed fluid is reduced, and the pressure in the returnline becomes nearly zero pressure, thereby minimizing energyconsumption.

Further, during normal driving, the predetermined pressure-maintainingvalve is completely opened so that the fluid is easily returned to thehydraulic oil tank, and if trouble in the system arises causing theengine to stop, the system renders the hydraulic lines the pressure formaintaining the vehicular height level smoothly, and especially even iftrouble occurs in the system during the driving mode, the pressurizedfluid from the pump is relieved at a lower pressure because the mainline pressure control valve is completely opened, thereby reducingenergy consumption and noise.

Further, the noise and vibrations, which bring about severe damage tothe performance of the relief valve and the entire hydraulic circuitsystem, is eliminated because the chattering phenomenon is prevented byinstalling a damping orifice in the relief valve by using a directmoving type with a simplified structure.

What is claimed is:
 1. An automotive active suspension system having abody and a plurality of wheels comprising:a hydraulic pressurecreating/supply apparatus; hydraulic actuators arranged between eachwheel and body; wheel pressure control valves for proportionallycontrolling the pressure applied to each hydraulic actuator; a main linepressure control valve arranged between said hydraulic pressurecreating/supply apparatus and said wheel pressure control valves tocontrol the main line pressure supplied through the main lines; apredetermined pressure-maintaining valve located in a return line tosaid hydraulic pressure creating/supply apparatus from said wheelpressure control valves to receive a pilot pressure passing through saidmain line pressure control valve; a relief valve located in a lineconnecting the upstream line of said main line pressure control valveand the downstream line of said predetermined pressure-maintainingvalve; a bypass line for connecting said main line pressure controlvalve to the downstream line of said relief valve; and means forcontrolling said main line pressure control valve wherein said hydraulicpressure creating/supply apparatus communicates with the main line inthe normal driving mode and communicates with said bypass line in thetrouble and engine stop modes.
 2. An automotive active suspension systemaccording to claim 1 wherein said predetermined pressure-maintainingvalve comprises a body, a sleeve arranged in the interior of said body,a member for sliding within said sleeve member and a poppet memberarranged in contacting position with one end of said sliding member sothat the front/rear wheel return lines communicate with the return line,a spring for elastically supporting the other end of said sliding memberand a member for regulating the elastic force of the spring in order toapply a predetermined force to said poppet.
 3. The automotive activesuspension system according to claim 2 where said popper member movesdownward so that the front/rear wheel return lines communicate with thereturn line when the total forces of the fluid pressure in the pilotline and the pressure in the front/rear wheel return lines exceed saidspring predetermined force.
 4. An automotive active suspension systemaccording to claim 1, wherein said predetermined pressure-maintainingvalve is opened in the normal driving mode, and is closed during thetrouble mode and the engine stop mode.
 5. An automotive activesuspension system according to claim 4, wherein said predeterminedpressure-maintaining valve starts to be open when the pressure of saidmain line becomes the pilot pressure.
 6. An automotive active suspensionsystem according to claim 1 wherein said main line pressure controlvalve comprises a body, a member capable of sliding in said body, asleeve arranged at the outside of said sliding member, a spring mountedon one end of the sliding member, the spring applying a substantiallyconstant force to said sliding member, and a plunger member positionedat the opposite end of said sliding member, said plunger member beingcontrolled by said controlling means.
 7. An automotive active suspensionsystem according to claim 6 wherein said plunger forces said slidingmember to a position whereby the main system line communicates with thefront/rear main lines during the normal driving mode and wherein theplunger moves in the opposite direction causing said spring member tomove said sliding member in the opposite direction whereby the main linecommunicates with said bypass line in the engine stopped and troublemodes.
 8. The automotive active suspension system according to claim 1wherein each of said wheel pressure control valves comprise a pilotvalve operated by said controlling means and a main valve, the pilotvalve comprising a body, an outer sleeve having a return orificecommunicating with first and second return lines, an inner sleeve havinga supply orifice communicating with first and second main lines, a pilotorifice for sending fluid to said main valve, a poppet adjusted to slidewithin said outer sleeve and a plunger operated by said controllingmeans to push the rear portion of said popper, said main valvecomprising a body, positioned adjacent to said pilot valve, a sleevemember mounted in said body and a spool member arranged to slide in saidsleeve member, said sleeve member having a port which communicates withfirst and second main lines, a port communicating with a control lineand a port communicating with said first and second return lines, saidspool member having a pilot chamber communicating with said pilotorifice in a first position, and with a return chamber in a secondposition, said return chamber containing a spring member and a returnorifice for connecting said return chamber to the exterior of said spoolmember.